The City’s State of the Coast Report (2007) identified that half of Cape Town’s dune systems are under serious threat. Natural factors, such as strong south-easterly and north-westerly winds during summer and winter respectively, and the sediment characteristics of sand along the Cape Town coastline (generally fine-grained and composed predominantly of low density quartz), coupled with inappropriate development within the coastal dune cordon, has caused migrating coastal dunes to become a hazard along parts of the coastline backed by low relief topography. Other aeolian (arising from the action of wind) transport hazards such as sand blasting and damage to various machinery or equipment through blockages are also present.
Dune migration can either occur through normal aeolian transport processes, or via the rollover model through sea-level rise, where erosion and deposition occur on the seaward and landward sides of the dune or sedimentary barrier respectively, causing transgression of the barrier feature. Climate change may also result in stronger wind regimes, which may also increase dune migration. Removal of natural dune vegetation or replacement with alien vegetation, sand mining, e.g. at Macassar, and the development of uncontrolled and unauthorised beach access tracks and pathways through dune cordons can destabilise dunes, increasing their susceptibility to erosion and migration. Damage to or changes in dune systems can also leave coastal regions vulnerable to other coastal hazards, as a result of them acting as natural barriers to storm damage, inundation and erosion.
As a result of the majority of the Atlantic Seaboard and a large portion of False Bay being dominated by rocky coastlines, migrating dunes are restricted to Table Bay and the West Coast, along with the sandy northern shores of False Bay (the exceptions being the pocket beaches of Hout Bay, Glencairn and Fish Hoek).
The sites where dune migration is considered a high present or potential future hazard include:
- Silwerstroom
- Blaauwberg Conservation Area
- Bloubergstrand
- Rietvlei
- Milnerton
- Hout Bay
- Glencairn
- Fish Hoek
- Muizenberg to Wolfgat
- Monwabisi/Macassar
- Eerste/Lourens River mouths
The areas where damaged dunes require rehabilitation and stabilisation, as identified in the City’s State of the Coast Report (2007), are:
- Blaauwberg Conservation Area
- Kommetjie
- Soetwater
- Bloubergstrand
- Milnerton
- Hout Bay
- Simon’s Town
- Glencairn
- Fish Hoek
- Muizenberg to Wolfgat
- Monwabisi/Macassar
- Strand
- Bikini Beach